Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Beyond the annotations, it discloses that results contain caller names and phone numbers (PII), that the tool reaches back only through the 500 most recent messages, that it requires lines:manage, and that it is rate limited per account. These are material behavioral facts not present in the structured annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is dense but front-loaded and well organized: purpose and ordering first, result fields next, then pagination, alternatives, and operational constraints. Every sentence adds actionable information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description supplies the returned item shape (id, at, name, phone, message, urgent), pagination termination behavior, retention limit, permission requirement, and related tools. Nothing an agent needs to call it correctly is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3, but the description adds pagination semantics beyond the schema: limit is page size, before should be the previous result's next_before, and next_before is null when no older messages exist. It does not add much about line_id or limit bounds beyond the schema, but the cursor-chaining explanation is valuable.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description opens with a specific verb and resource ('List the messages the answering line took from callers for one of the signed-in owner's own lines, newest first') and explicitly distinguishes the tool from get_line_calls and get_replies. An agent can identify exactly what this returns without inspecting the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It states where line_id comes from ('Pass the line_id from get_answering_line'), explains how to paginate via next_before and before, and names the sibling tools that cover calls and Direct replies. When to use this versus alternatives is clear.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.